Continous electrolytic pickling method for metallic products using alternate current supplied cells

ABSTRACT

Continuous electrolytic pickling method for steels, Nickel, superalloys, Titanium and alloys thereof, characterised in that the material to be treated , for a time comprised between 3 sec and 60 sec, is immersed into or passes through at least one electrolytic cell with an electrolytic solution consisting of a neutral or acid aqueous solution, at a temperature comprised between 20 DEG C. and 95 DEG C., with at least one pair of electrodes connected to an alternate current power supply having a frequency ranging from 1 Hz to 1000 Hz, the electrolysis being carried out at a current density having an effective amplitude ranging from 10 A/dm2 to 250 A/dm2. The Figure depicts the progress of the weight loss of an AISI 409 (X6CrTil2) steel as a function of the application time of an embodiment of the pickling method according to the present invention.

DESCRIPTION

[0001] The present invention relates to a continuous electrolytic pickling method for metallic products, in particular in Iron, Nickel, Titanium and alloys thereof, based on the use of alternate current supplied cells with electrolytic solution consisting of acid or neutral aqueous solutions.

[0002] As it is known, pickling, e.g. for the stainless steels, is actually carried out in order to eliminate the scale of thermal oxides which are generated during the hot-rolling and/or annealing treatments, and to dissolve the Chromium-depleted alloy layer (dechromized layer) therebelow. This conventional method consists of three conceptually different steps: a first step of descaling, i.e., physico-chemical modification of the scale with the partial removal thereof; a second step of actual pickling, i.e. removal and solution of the residual scale and disposal of the sub-surface layer of dechromized alloy; and a third step, so-called of finishing, consisting in a surface passivation. The latter two steps are often carried out at the same time. In the state of the art, several ways for carrying out the step of descaling, depending on the type of oxide scale present in the metal at the end of metallurgical treatments, are known.

[0003] With regards to the oxide scale generated in the hot-rolling and annealing processes, the step. of descaling, in most cases, is carried out by sandblasting with a hard grit which breaks up and partially removes the scale. For the cold-rolled products and the annealed stainless steel and/or Titanium, the step of descaling cannot be carried out by surface peening, which is not compatible with the quality of the finished product's surface. Hence, different processes, capable of inducing a substantial modification of the oxides, facilitating the subsequent pickling process, are adopted.

[0004] To this end, the most widely adopted methods are as follows:

[0005] a) thermochemical descaling, which consists in immersing the material to be pickled in a bath of melted oxidizing salts (400° C.-600° C.) capable of altering the scale, increasing the degree of oxidation of the metals constituting the oxides. In particular, Kolene baths (eutectic of the NaOH-NaNO₃-NaCl ternary system) at temperatures around 500° C. are the most widely adopted;

[0006] b) electrolytic descaling by neutral sulphate solutions, with the partial modification of the oxidation states of the constituent metals of the scale and the entailed solution thereof.

[0007] For both hot- and cold-rolled stainless steels and Titanium, the pickling step is generally carried out using highly oxidizing acid baths, capable of dissolving the sub-surface alloy layer (Cr-depleted for stainless steels) determining the detachment of the scale adhering thereto.

[0008] These baths mainly consist of mixtures of mineral acids, the most widely adopted thereamong being:

[0009] 1) mixtures of nitric and hydrofluoric acid at temperatures generally between 60° C. and 75° C.;

[0010] 2) mixtures of sulphuric, hydrofluoric, hydrochloric and phosphoric acids, with additions of elements having a high oxidizing power (sometimes used in mixtures) like, e.g., permanganates, persulphates, ferric chloride, ferric sulphate and hydrogen peroxide, at a temperature between 50° C. and 100° C.;

[0011] 3) hydrochloric or sulphuric acid with the addition of corrosion inhibitors for the pickling of unalloyed steels at temperatures comprised between 50° C. and 85° C.

[0012] In some industrial plants, in order to increase the kinetics of the pickling step with all of the abovementioned mixtures, electrolytic cells capable of applying a continuous current density ranging from 3 A/dm² to 40 A/dm² to the material are used.

[0013] The finishing step, aimed at forming a passivated protective film, when this is not carried out at the same time as the pickling step, is usually attained in baths having high redox potential. These baths contain the abovementioned acids and oxidizers at lower concentrations and with a lower content of the ions of the metals present in the metallic products to be pickled.

[0014] In general, between each tank, and anyhow at the end of the line, washing sections consisting of water jet systems equipped with rotary brushes are inserted. The function carried out by these systems is the removal of the pickling solution dragged by the strip, and the non-adhering scale particles deposed on the strip surface.

[0015] To date, several processes, concerning the descaling step as well as the pickling step of stainless steels and of Titanium and alloys thereof, based on the employ of acid solutions free from nitric acid, are known. In particular, processes for the pickling of stainless steels and of Titanium and alloys thereof based on the use of acid solutions, free from nitric acid, whose oxidizing power is provided by the presence of various elements, among which the ferric ions, the hydrogen peroxide, and the persulphates, are known.

[0016] In particular, DE-A-19624436, WO 9826111, EP-A-763609 and JP95-130582 disclose pickling processes in acid solution, free from nitric acid, in presence of ferric ions with the use of alternate current supplied electrolytic cells (current density comprised between 0.5 A/dm2 and 250 A/dm2). DE-C-3937438 discloses a process in which direct current is employed for the reoxidation of the ferrous ions to ferric ions in a hydrochloric acid solution.

[0017] EP-A-838542 discloses a pickling process in an aqueous sodium sulphate solution, having a concentration between 10 g/l and 350 g/l, in which the strip is vertically passed between pairs of counter electrodes, a direct current having a density between 20 A/dm2 and 250 A/dm2 being applied therebetween.

[0018] However, the known technologies, which are schematically reported hereto, entail significant environmental and working safety drawbacks, as well as drawbacks referring to the management of the pickling process in terms of control and costs thereof.

[0019] With regards to the descaling step, the main drawback of the process for the mechanical removal of the scale by sandblasting or peening lies in the difficulty of abating the dusts made of silica and metallic oxides particles, not to mention the high noise pollution of the surrounding working areas.

[0020] The chemical descaling carried out with melted salts proves particularly difficult to manage, due to the high temperature of the bath (400° C.-600° C.) as well as to the difficulty of disposing the washing solutions of the metallic product to be descaled at the end of the treatment. In fact, these washing solutions contain non-negligible quantities of hexavalent Chromium and of nitrites and nitrates.

[0021] The employ of baths containing nitric acid, for the pickling and the finishing steps, causes relevant environmental problems, for different reasons. Among the latter, the most important are:

[0022] A. difficulty of abating the highly polluting NO_(x)s, evolved from the acid-metal reactions;

[0023] B. difficulty of complying with the existing norms for the environmental protection, for the disposal of spent solutions in connection with the high nitrate content thereof.

[0024] The baths of sulphuric and of hydrofluoric acid which use, instead of the nitric acid, systems having a high is redox potential, entail a complex management in connection with the difficulties of maintaining the appropriate concentrations of reagents capable of ensuring the envisaged pickling kinetics.

[0025] Moreover, the costs of the reagents, e.g., of the stabilised hydrogen peroxide, are high, considering that a fraction of the metals accumulating in the solution react with the oxidizers, lowering the process effectiveness.

[0026] The present invention allows to overcome all of the abovementioned drawbacks, further providing other advantages which will hereinafter be made apparent.

[0027] In particular, an object of the present invention is to provide a pickling method for continuously cast products in steel, in compliance of the UNI EU 74/20 norm, and in Nickel alloy and in Titanium, based on the use of alternate current supplied electrolytic cells in acid or neutral aqueous solutions.

[0028] In fact, an object of the present invention is a continuous electrolytic pickling method for steels, Nickel super alloys, Titanium and alloys thereof, characterised in that the material to be treated, for a time between 3 and 60 seconds, is immersed into or passes through at least one electrolytic cell with an electrolytic solution, free of nitric acid, consisting of a neutral or acid aqueous solution, at a temperature between 20° C. and 95° C., with at least one pair of electrodes connected to an alternate current power supply having a frequency ranging from 40 Hz to 70 Hz, the electrolysis being carried out at a current density having an effective amplitude ranging from 10 A/dm2 to 250 A/dm2.

[0029] The electrolytic solution can be an aqueous solution, at a temperature between 20° C. and 95° C., containing the following components having a concentration expressed in g/l:

[0030] sulphuric acid (H₂SO₄) from 20 to 300, and at least one among

[0031] hydrofluoric acid (HF) from 5 to 50

[0032] orthophosphoric acid (H₃PO₄) from 5 to 200

[0033] ferric ions (Fe⁺³) from 5 to 40.

[0034] In the case of pickling of stainless steels, the electrolytic solution is maintained at a temperature between 70° C. and 90° C. and comprises sulphuric acid at a concentration between 150 g/l and 250 g/l, and ferric ions (Fe⁺³) at a concentration of from 5 g/l to 40 g/l.

[0035] In the case of Nickel-base super alloys and for Titanium and alloys thereof, the electrolytic solution is at a temperature between 70° C. and 90° C., and comprises sulphuric acid at a concentration between 150 g/l and 250 g/l and at least one between hydrofluoric acid at a concentration between 5 g/l and 50 g/l and hydrochloric acid at a concentration between 5 g/l and 50 g/l.

[0036] For carbon steels, the electrolytic solution is at 70° C.-90° C. and comprises sulphuric acid at a concentration between 150 g/l and 250 g/l.

[0037] In another embodiment, for any application, the electrolytic solution may be a sodium sulphate (Na₂SO₄) neutral aqueous solution having a concentration ranging from 25 g/l to 300 g/l at a temperature between 500° C. and 95° C.

[0038] In an embodiment of the present invention, pairs of adjacent electrodes are connected to two separate power supplies, so that the current lines outputted from a first electrode pair facing one side of the material to be treated, cross said material and close again on a second electrode pair, opposed to the first one and facing the other side of the material to be treated, defining a substantially X-shaped course.

[0039] In another embodiment of the present invention, electrodes facing one side of the material to be treated are connected to a power supply, so that the current lines which are outputted from said electrodes and cross the material, close again on other electrodes opposed to the first ones and facing the opposite side of the material to be treated, defining a course which is substantially orthogonal to said sides.

[0040] The electrolytic pickling method according to the invention can be used for inducing a physical-chemical modification of the scale of the metallic oxides present onto the surface of the material to be pickled, a physical-chemical modification which, in the case of the stainless steels, occurs in a treatment time comprised between 1 sec and 10 sec.

[0041] The continuous electrolytic pickling method according to the present invention may be used in a step subsequent to that of the physical-chemical modification of the scale of metallic oxides present onto the surface of the material to be pickled. In the case of the stainless steels, this application of the pickling method according to the invention requires treatment times between 2 sec and 15 sec.

[0042] The electrolytic cells employable in the present invention may be vertical electrode or horizontal electrode cells, the former ones being preferable for the easy scavenging of the gas evolved by electrochemical reaction at made circuit.

[0043] The electrodes are made in materials resistant to the corrosive action of the baths employed.

[0044] The electrodes in the individual cell are connectable according to at least three schemes, reported by way of a non-limiting example in the attached FIGS. 2, 3 and 4.

[0045] In the case of opposed electrodes connected to the same terminal, the electrodes may consist of an individual toroidal ring.

[0046] Another object of the present invention are the electrolytic cells characterised in that they have an electrode connection as indicated hereinafter and claimed in claims 8 and 9.

[0047] Among the most important advantages in the adoption of the present invention, the following may be mentioned:

[0048] provision of a descaling system capable of replacing currently employed technologies, minimising the problems connected to pollution and working safety;

[0049] provision of a pickling and finishing system to be carried out subsequently to the descaling treatment carried out with the currently adopted technologies, based on the use of solutions free from nitric acid capable of eliminating the environmental drawbacks connected to NO_(x)s emissions;

[0050] provision of a pickling method capable of reuniting in a single stage the descaling, pickling and finishing steps, carrying out the entire pickling process with a single treatment system;

[0051] provision of a pickling system capable of, significantly reducing the times, and therefore the costs, of the treatment.

[0052] The positive effects of the adoption of the continuous pickling method according to the present invention may be explained in light of the following: the AC flow induces an over voltage of the free corrosion potential on the surface of the alloy to be pickled, so as to reach thereon electrochemical potentials capable of fostering several oxidation-reduction reactions which involve both the alloy and the oxide layer thereabove, as well as the aqueous solution.

[0053] Hence, the change in the oxidation state of the metals present in the surface oxides (in particular, Chromium for stainless steels) and the solution of the underlying metal are carried out. Moreover, water electrolysis, with an intense production of Hydrogen and Oxygen, is carried out

[0054] These reactions, in increasing order of standard potential of the respective redox pairs, are:

[0055] oxidation reactions of the constituent metals of the steel or of the Titanium and alloys thereof, or of the Nickel and alloys thereof

Ti→Ti²⁺+2e

Cr→Cr³⁺+3e

Fe→Fe²⁺+2e

Ni→Ni²⁺+2e

[0056] reactions involving the constituent metals of the thermal oxide scale of the alloy to be pickled

Fe²+→Fe³⁺+e

Ti²+→Ti³⁺+2e

Ti³+→Ti⁴⁺+e

Cr³+→Cr⁶⁺+3e

[0057] water electrolysis reactions in acid solutions

4H₃O+4e→2H₂+4H₂O

6H₂O→O₂+4H₃O⁺+4e

[0058] in neutral solutions

4H₂O+4e→2H₂+4OH⁻

4OH⁻→O₂+2H₂O+4e

[0059] The generation of chromates (hexavalent Cr), by oxidation of the Chromium constituting the oxide scale, contributes to increasing the solution kinetics of the alloy and the oxidation of the ferrous ion to ferric ion according to the reactions

Cr₂O₇ ²⁻+6Fe²⁺+14H⁺→2CR³⁺+6Fe³⁺+14H₂O

[0060] The dissolved Iron, in form of ferrous ion deriving from the solution of steel, is capable of fostering the reduction, according to the abovedescribed reaction, of all the Cr (VI) ions to Cr (III) ions, so that the Cr (VI) ions be absent from the solution.

[0061] For the stainless steels, the presence in the solution of the redox pair consisting of ferric and ferrous ions (E^(o)=771 mV/SHE) elevates the oxidizing power of the bath, providing the latter with passivating capabilities.

[0062] The voltage-current phase displacement, assessed by electrode impedance spectroscopy, is such that at frequencies between 40 Hz and 70 Hz more than 90% of the current crossing the cell is in phase with the applied voltage (active component of the current) and it allows the abovementioned electrochemical reactions. A mere 10% of the current is shifted 90° out of phase with respect Is to the voltage (reactive component of the current) it being employed, for the load, the discharge of the pseudo-condenser made by the double electric charge layer on the electrode surface. As the frequency increases, the active component tends to decrease in favour of the reactive one, decreasing the fraction of the current fostering the electrochemical reactions required for the pickling.

[0063] The alternation of Hydrogen-developing reactions, during the cathode polarization, and of Oxygen-developing reactions, during the anodic polarization, yields an intense descaling action, causing the quick detachment of the oxide layer from the matrix.

[0064] The solution kinetics of the alloys to be pickled are high, particularly so, considering that the oxide is in no way pre-treated or conditioned prior to the in-cell electrolytic treatment.

[0065] So far, merely a general description of the present invention has been given. With the aidance of the figures and of the examples, having an explanatory yet not a limitative value, a more detailed description of specific embodiments thereof, aimed at making its objects, features, advantages and operation modes better understood, will hereinafter be provided.

[0066]FIG. 1 shows the progress of the weight loss expressed in g/m² as a function of the time for the entire pickling treatment in a 200 g/l H₂SO₄ aqueous solution for a X₆CrTi₁₂ (AISI 409) cold-rolled and annealed steel.

[0067]FIG. 2 shows a first electrode configuration, in which electrodes 1 located at the same side of the strip N are alternately connected to the two terminals of a phase of a transformer. With this configuration, the electrodes 2 located at the side opposed to the strip are connectable to the same terminal of a phase of a transformer, connecting them to the corresponding ones on the other side.

[0068]FIG. 3 shows a second configuration in which the electrodes 1 located in the same side of strip N are connected to the same terminal of one or more phases of one or more transformers, and the opposed electrodes 2 are connected to the other terminal of the corresponding phases of the transformers.

EXAMPLE 1

[0069] Pickling of a Strip (Coil) of Cold-rolled and Annealed AISI 409LI

[0070] The characteristics of the strip to be pickled are: strip width 1270 mm strip thickness 1.5 mm coil weight 18900 kg coil length 956 m Pickling solution: H₂SO₄ concentration 200 g/l solution temperature 60° C. ± 5° C.

[0071] The current is applied with the electrode configuration shown in FIG. 2. The interelectrode gap is equal to 80 mm. At the outlet of the electrolytic cells a water jet system equipped with brushes, followed by a series of wringing rolls, was inserted. The in-tank solution is such as to maintain with the immersed strip surface a ratio not lower than 1 m³ solution/m³ strip and it is renewed upon reaching the limits set for the dissolved metals.

[0072] Assessing the loss due to currents closing again between the electrodes without involving the strip to be pickled to be <8%, a current density equal to 60 A/dm² was set. The treatment time, i.e., the period in which the material is subjected to the action of the alternate current, was set at 15 sec, in connection with the strip speed and the electrode sizes.

[0073] The weight loss attained at the end of the treatment was equal to about 40 g/m² of strip.

[0074]FIG. 1 shows the diagram related to the weight losses of the steel subject of the example as a function of the treatment time and for two different current densities applied.

EXAMPLE 2

[0075] Pickling According to the Invention of a Cold-rolled and Annealed AISI 430 Strip (Coil)

[0076] The characteristics of the strip to be pickled are: strip length 1270 mm coil thickness 1.0 mm coil weight 18900 kg coil length 1907 m Pickling solution: H₂SO₄ concentration 250 g/l solution temperature 60 ± 5° C.

[0077] The current was applied with the electrode configuration shown in FIG. 2. The interelectrode gap is equal to 80 mm. At the outlet of the electrolytic cells a water jet system equipped with brushes, followed by a series of wringing rolls, was inserted. The in-tank solution is such as to maintain with the immersed strip surface a ratio not lower than 1 m³ solution/m² strip and it is renewed upon reaching the limits set for the dissolved metals.

[0078] Assessing the load loss due to currents which close again between the electrodes without involving the strip to be pickled to be <8%, a current density equal to 75 A/dm2 was set.

[0079] The treatment time, i.e., the period in which the material is subjected to the action of the alternate current, was set at between 5 sec and 25 sec, in connection with the strip speed and the electrode sizes The weight loss attained at the end of the treatment was equal to about 40 g/m² of strip. 

1. A continuous electrolytic pickling method for steels, Nickel super alloys, Titanium and alloys thereof, characterised in that the material to be treated, for a time comprised between 3 sec and 60 sec, is immersed or travels through at least one electrolytic cell with an electrolytic solution, free from nitric acid, consisting of a neutral or acid aqueous solution, comprising sulphuric acid from 20 to 300 g/l at a temperature comprised between 20° C. and 95° C., with at least one pair of electrodes connected to an alternate current power supply having a frequency ranging from 40 Hz to 70 Hz, the electrolysis being carried out at a current density having an effective amplitude ranging from 10 A/dm2 to 250 A/dm2.
 2. The electrolytic pickling method according to claim 1, wherein the electrolytic solution is an aqueous solution, at a temperature comprised between 20° C. and 95° C., containing the following components having concentrations expressed in g/l: sulphuric acid (H2SO4) from 20 to 300, and at least one among hydrofluoric acid (HF) from 5 to 50 orthophosphoric acid (H₃PO₄) from 5 to 200 ferric ion (Fe⁺³) from 5 to
 40. 3. The electrolytic pickling method for stainless steels according to any one of the claims 1 to 2, wherein the electrolytic solution is maintained at a temperature between 70° C. and 90° C. and comprises sulphuric acid at a concentration comprised between 150 g/l and 250 g/l, and ferric ions (Fe⁺³) at a concentration of from 5 g/l to 40 g/l.
 4. The electrolytic pickling method for Nickel-base super alloys and for Titanium and alloys thereof according to claims 1 to 2, wherein the electrolytic solution is maintained at a temperature between 70 and 90° C. and comprises sulphuric acid at a concentration between 150 g/l and 250 g/l and at least one between hydrofluoric acid at a concentration between 5 g/l and 50 g/l and hydrochloric acid at a concentration between 5 g/l and 50 g/l.
 5. The electrolytic pickling method for carbon steel according to any one of the claims 1 to 2, wherein the electrolytic solution is maintained at 70° C.-90° C. and comprises sulphuric acid at a concentration between 150 g/l and 250 g/l.
 6. The electrolytic pickling method according to claim 1, wherein the electrolytic solution is a sodium sulphate (Na₂SO₄) aqueous solution having a concentration ranging from 25 g/l to 300 g/l at a temperature between 50° C. and 95° C.
 7. The electrolytic pickling method according to any one of the claims 1 to 6, wherein pairs of adjacent electrodes are connected to two separate power supplies, so that the current lines, outputted from a first electrode pair facing one side of the material to be treated, cross said material and close again on a second electrode pair, opposed to the first pair and facing the other side of the material to be treated, defining a substantially X-shaped course.
 8. The electrolytic pickling method according to any one of the claims 1 to 6, wherein electrodes facing one side of the material to be treated are connected to a power supply, so that the current lines, which are outputted from said electrodes and cross the material, close again on other electrodes opposed to the first ones and facing the opposite side of the material to be treated, defining a course which is substantially orthogonal to said sides of the material to be treated.
 9. A use of the electrolytic pickling method according to claims 1 to 8, for inducing a physical-chemical modification of the scale of the metallic oxides present onto the surface of the material to be pickled.
 10. The use of the electrolytic pickling method according to claim 9, for stainless steels, with a treatment time between 1 and 1.0 sec.
 11. The use of the electrolytic pickling method according to claims 1 to 8, in a step subsequent to that of the physical-chemical modification of the scale of metallic oxides present onto the surface of the material to be pickled.
 12. The use of the electrolytic pickling method according to claim 11—in a step subsequent to that of the physical-chemical modification of the scale of metallic oxides present onto the surface of the material to be pickled—for stainless steels, with a treatment time comprised between 2 sec and 15 sec.
 13. The use of the pickling method according to claims 1 to 12, combined to other conventional pickling systems.
 14. Electrolytic cells, characterised in that they have an electrode connection as indicated in claim 7 or
 8. 